Here’s what you’ll learn when you read this story:

• It used to be thought that only one species of man-o-war jellyfish lived in Japanese waters, but a stranded specimen ended up being an entirely new species.
• Physalia mikazuki, whose name translates to “crescent helmet,” was named in honor of the crescent moon on the helmet of samurai warrior and feudal lord Date Masamune.
• There are now concerns about global warming making it possible for these jellyfish to survive further north, which can be hazardous to swimmers and the ecosystem.

Washed up on the shore of Gamo Beach in northeast Japan, a gelatinous bluish blob caught the eye of researcher Yoshiki Ochiai. It appeared to be the remains of a jellyfish left behind by receding waves. Ochiai, who had never seen such a creature before, collected the tangle of tentacles in a plastic bag and headed straight to the lab of Tohoku University.

It turned out that this creature was a species of man-o-war that had never been identified before. Otherwise known by the name of their genus, Physalia, these cnidarians are not true jellyfish, but more closely related to siphonophores—colonial creatures in which groups of organisms, or zooids, perform functions that power the entire animal.

The new species was named Physalia mikazuki (“crescent helmet man-o-war”) after the crescent moon that crowned the helmet of legendary samurai warrior and powerful Edo-period feudal lord Date Masamune.

Only four other species of man-o-war were known before this discovery. Physalia utriculus floats along the waves off the coast of Okinawa all the way to Saganami Bay, and was assumed to be the only local Physalia species. It is now thought that P. mikazuki had remained unknown for so long because it frequents the same waters.

The beach where the P. mikazuki specimen was found is the furthest north that Physalia—its gas-filled pneumatophore usually adrift in tropical regions—has ever been seen. These creatures rely on winds and currents to carry them across the ocean as their tentacles passively snare unfortunate prey.

The pneumatophore of P. mikazuki was found to be morphologically different from P. mutriculus and the three other known Physalia species, P. physalis, P. megalista, and P. minuta. Besides its impressive helmet, some features of P. mikazuki that distinguish it from other species include having more than one primary tentacle and yellow, banana-shaped gastrozooids, which capture and digest food. Genetic analysis also confirmed that P. mikazuki is a distinct species.

“This is the first record of Physalia in Tohoku, Japan, a region historically outside the genus’s known range,” the researchers said in a 2025 study published in the journal Frontiers in Marine Biology and Ecology. “With Physalia reports previously limited to the warmer temperate waters of Sagami Bay, and subtropical Okinawa, the emergence of P. mikazuki in Sendai Bay highlights a significant biogeographical shift, raising important questions about the ecological implications.”

Migrations of these creatures were analyzed by running a simulation that uses data from sightings to estimate where they will end up in the short and long term. Sure enough, the simulation showed that P. mikazuki journeyed north from Saganami Bay in central Japan until it reached the same beach Ochiai found the stranded specimen.

Rising temperatures seem to be creating a wider habitable zone for them, something similar to what happened with the Nomura jellyfish, which is now taking over the waters surrounding Japan and threatening both the ecosystem and the fishing industry.

The painful (and sometimes fatal) sting of a man-o-war is hazardous to swimmers, which means there should be heightened public awareness and more safety precautions taken on Japanese beaches. The researchers think that monitoring the migrations of both P. mikazuki and P. utriculus will assuage safety concerns, help them find out more about how these floating carnivores influence the ecosystems they populate, and show them whether any more new species are hiding among the masses.

“These jellyfish are dangerous and perhaps a bit scary to some,” researcher Ayane Totsu said in a recent press release, “but also beautiful creatures that are deserving of continued research and classification efforts.”